The Three Lions progressed to the last eight with a comfortable 3-0 win over Senegal on Sunday.
France have won three of four games at this World Cup, suffering only a shock 1-0 defeat to Tunisia after rotating their first team having qualified for the knockout stage.
Kylian Mbappe has been in fine form this tournament, with five goals in four games already, firming the world champions up as one of the title favourites.
“It’s the biggest test we could face,” Southgate said.
“They are world champions, have an incredible depth of talent and outstanding individual players that are very difficult to play against. It’s a great challenge.”
France swept aside Poland 3-1 in their last-16 clash, continuing their fine run despite the absence of injured stars like Karim Benzema, Christopher Nkunku, Theo Hernandez, Paul Pogba and N’Golo Kante.
Southgate acknowledged the threat of France headlined by Mbappe but pointed to their depth of options.
“(Mbappe’s) a world-class player, he’s already delivered big moments in this tournament and other tournaments,” Southgate said.
“They also have (Antoine) Griezmann who has played over 70 consecutive games for France, he’s a phenomenal player.
“We know Olivier Giroud so well, and they have outstanding young midfield players.”